Introduction
Ever had to lift a brand-new floor because a pipe clash forced a last-minute notch? It’s a horrible way to lose a day and your margin. This guide shows you exactly how to plan, cut, and reroute services through floor joists without weakening the structure or inviting callbacks. We’ll cover the practical rules most UK building control officers expect, step-by-step planning, tooling, and when to reinforce or walk away. You’ll also see how to document decisions cleanly so clients sign off on the chosen route before you drill a single hole.
Table of Contents
Key Takeaways
- In general, notches go in the top of solid timber joists only, between 0.07L and 0.25L from a support, and no deeper than 12.5% of the joist depth.
- Commonly, holes in solid timber go on the joist centreline, between 0.25L and 0.4L from supports, with diameters no more than 25% of joist depth and spaced at least 3 diameters apart.
- Engineered I-joists: never cut flanges; only cut webs within manufacturer templates. Metal web joists allow generous web space but still follow maker limits.
- Planning ahead typically saves 2–3 hours per small bathroom or kitchen reroute by avoiding clash rework and floor re-lifts.
- Document the route with photos, dimensions, and client acceptance before cutting; many contractors find this halves disputes later.
Why Joist Cuts Go Wrong (And How To Avoid It)
The Problem
Many contractors discover clashes late: a 40 mm waste where a cable bundle already runs, or a soil vent stack that wants mid-span. Under pressure, someone notches too deep near a bearing or drills a big hole off-centre. In general, that can reduce joist capacity more than you think, and it’s common for remedial sistering to cost a half to full day including lifting boards and making good.
The Solution
- Use simple span-based zones to mark where notches and holes are allowed.
- Choose hole size and notch depth by the joist depth, not the pipe size first.
- Route services in groups and use drop ceilings or boxing when the numbers don’t add up.
- Record decisions (route, sizes, zones) and get client sign-off before cutting.
Real-World Example
A 2.9 m span of 195 mm deep C24 joists over a ground floor renovation needed a 32 mm basin waste and two 25 mm conduits. Planning took 20 minutes: zones were chalked, holes centred on the joist neutral axis, spaced 3x diameter. No mid-span notches, no edge tear-out. Result: zero rework, and the team saved what’s commonly 1–2 hours of “find and fix” on day two.
The Rules Of Thumb You Can Rely On
These are widely used UK site rules for solid timber joists and align with what building control commonly expects. Always follow manufacturer instructions for engineered products and get an engineer when outside these limits.
Solid Timber Joists (Softwood)
- Notches:
- Position: top of joist only, between 0.07L and 0.25L from a support (L = span).
- Depth: max 0.125 Ă— joist depth.
- Never notch the bottom of the joist. Never notch over supports or in mid-span.
- Holes:
- Position: centreline of the joist (mid-depth), between 0.25L and 0.4L from supports.
- Diameter: max 0.25 Ă— joist depth.
- Spacing: at least 3 Ă— diameter between hole edges.
- Edge cover: leave at least 50 mm from the top and bottom edges.
In general, sticking to these ranges maintains capacity and stiffness well enough for typical domestic loads without triggering structural checks.
Engineered I-Joists
- Never notch or drill flanges. That’s a structural red line.
- Holes are usually permitted in the web only, within printed or downloadable templates from the manufacturer.
- Typical limits: minimum distance from supports and maximum hole size depend on depth and series—always check the maker’s chart. If in doubt, ask for a hole-location approval sketch.
- Designed for services; however, large ducts/pipes still have limits.
- Keep clear of end bearings and follow the manufacturer’s brace and web-cut guidance.
- Don’t remove or weaken any metal webs; use specified knockout positions where provided.
Quick Comparison
| Joist Type | Where Holes/Notches Allowed | Typical Limits | Notes |
|---|
| Solid Timber | Notches top only (0.07L–0.25L); holes on centreline (0.25L–0.4L) | Notch ≤ 12.5% depth; hole ≤ 25% depth; holes 3D apart | Never notch bottom. Keep 50 mm from edges. |
| I-Joist | Web holes only per template | Sizes and setbacks vary by series | Never cut flanges. Get maker approval when unsure. |
| Metal Web | Between webs within guidance | Varies by depth and web pattern | Great for services; still respect end bearing zones. |
Step-By-Step: Plan A Safe Service Route
1. Identify Joist Type, Span And Depth
- Measure span (support to support) and joist depth. Note joist type: solid timber, I-joist, or metal web.
- Mark the allowed zones on the floor with a pencil/chalk line. Many contractors find this 10-minute step prevents most mistakes.
Data Point
In general, a 3 m span with 200 mm solid timber joists gives a hole zone between 0.75 m and 1.2 m from each support (0.25L–0.4L), with hole diameters up to roughly 50 mm (0.25 × 200 mm).
2. Group Services And Choose The Right Path
- Bundle small cables together and keep them on the joist centreline where possible.
- For 32–40 mm wastes, plan a series of holes within the allowed zone or consider a drop ceiling if alignment pushes holes into forbidden areas.
- Large services (e.g., 110 mm soil) should not go through joists mid-span. Use trimmers around an opening, reroute, or box.
3. Size Holes/Notches From The Joist, Not The Pipe
- Start with allowable hole or notch size from the rules. If your service is bigger—don’t force it. Change route or detail.
- Leave small clearance: commonly 3–5 mm around pipes to avoid squeaks and accommodate expansion.
4. Mark, Photograph, And Get Approval
- Mark hole centres and notch extents. Photograph with a tape in shot and note the span reference.
- Use Donizo to capture the route by voice with photos while you’re on site, then generate a clear proposal with those notes. Clients can e‑sign the acceptance before any cutting.
Data Point
Contractors often report that documenting the agreed route reduces back-and-forth by roughly half and saves 2–3 hours across a small kitchen or bathroom fit.
5. Cut Cleanly, Check, And Protect
- Drill pilot points, use a holesaw for clean edges, and back the cut to limit breakout.
- For notches, use a fine-tooth saw and sharp chisel; keep square shoulders—no overcutting.
- Seal or grommet where required, and protect edges from abrasion (especially for cables).
6. Final Inspection And Photo Log
- Confirm hole locations and sizes against your marks.
- Photograph each run before closing the floor. Add the photos to your Donizo proposal file; if the client later queries, you’ve got clear evidence.
Drilling And Notching Techniques That Prevent Damage
Solid Timber: Clean Holes On The Neutral Axis
- Aim for the true centreline of the joist depth—this is the neutral axis where bending stress is lowest.
- Use hole saws for 25–60 mm. For smaller holes (16–22 mm), a brad-point bit gives cleaner exits than a spade bit.
- Keep holes at least 50 mm from the top and bottom edges.
Example
A 47 × 195 joist: maximum hole diameter about 48 mm. Two 32 mm wastes? Stagger in adjacent joists or plan a short drop ceiling—don’t oversize one hole to cram both.
Notches: Short, Shallow, Square
- Only at the top, only within 0.07L–0.25L from supports, and no deeper than 12.5% depth (about 24 mm on a 195 mm joist).
- Never taper beyond the allowed length, and avoid feathered ends—square shoulders carry load better.
I-Joists: Template Or Engineer, Nothing Else
- Web holes only, within the printed template ranges. If your position or size falls outside, stop and get written manufacturer guidance.
- Never notch the flange to “gain a few millimetres.” That invites serious structural failure.
- Route within the open web; avoid cutting metal members.
- Maintain manufacturer edge distances to bearings and bracing nail plates.
Data Point
It’s common for flooring re-lifts and remedial sistering after an over-notch to take 0.5–1 working day including making good—time you can avoid with 15–30 minutes of planning and clean, compliant cuts.
When You Must Reinforce Or Reroute
Signs You’re Outside The Safe Envelope
- The service diameter exceeds the allowed hole size (e.g., 65 mm through a 170 mm joist).
- The only viable location is outside the permitted zones (e.g., mid-span notch request).
- Multiple holes line up, leaving less than 3 diameters spacing.
Reinforcement Options (Solid Timber)
- Sistering: bolt and glue a full-depth timber alongside the affected joist, extending well beyond the hole/notch zone. In general, aim for at least the middle third of the span or full length if practical.
- Trimmers and headers: frame an opening and transfer load to adjacent joists with double members and hangers.
- Engineer plates: proprietary reinforcement plates can restore capacity for specific hole sizes/positions—engineer sign-off required.
When To Reroute Instead
- Large pipes (e.g., 110 mm) mid-span: choose boxing, a bulkhead, or move the stack.
- I-joist flange conflicts: reroute—do not touch the flange. Maker-approved web holes only.
Example: Bathroom Waste Clash
A 40 mm shower waste wants to cross three 220 mm solid joists near mid-span. Safe hole diameter is up to 55 mm, but the allowed hole zone is 0.25L–0.4L. The trap centreline sits at 0.5L—no-go. The team drops the ceiling 75 mm for a 1.2 m run, keeps all holes within the allowable zone thereafter, and finishes on programme. In general, that decision saves what commonly becomes a full day of rework if discovered late.
Frequently Asked Questions
How Big Can I Drill A Hole In A Solid Timber Joist?
In general, no more than 25% of the joist depth, positioned on the centreline, and between 0.25L and 0.4L from supports. Space holes at least three diameters apart and keep at least 50 mm from the top and bottom edges. If you need bigger, reroute or seek engineering advice.
Can I Notch The Bottom Of A Joist For A Pipe?
No. Notches belong at the top only and only within 0.07L–0.25L from supports. Bottom notches significantly reduce tensile capacity and are a frequent cause of bounce and cracking below.
What About Engineered I-Joists—Can I Cut Them?
Never cut the flanges. Holes are often allowed in the web within strict manufacturer templates. If your proposed hole or position is outside the template, get written approval or redesign the route. Cutting flanges voids warranties and can be dangerous.
How Do I Fix An Over-Notched Joist?
Commonly, you’ll sister the joist: fix a matching-depth timber alongside with structural adhesive and bolts or coach screws, extending well past the notch zone. In many cases, building control will want an engineer’s note. Don’t rely on small plates or filler—restore the section properly.
Do I Need Client Permission Before Cutting?
It’s smart practice. Mark the route, sizes, and any visible boxing or drops. Share photos and a short explanation, then get written acceptance. With Donizo, you can generate a professional proposal from voice, text, and photos on site, send it for e‑signature, and convert to an invoice when accepted.
Conclusion
Joist drilling and notching isn’t complicated—until it goes wrong. Work to the simple span-based zones, size from the joist not the pipe, and either reinforce correctly or reroute when the numbers don’t fit. Document the route before you cut; it protects your programme and your margin. If you want to tighten this up, use Donizo to capture your plan by voice with photos, generate a clean proposal your client can e‑sign, and convert the accepted scope to an invoice in one click. Less admin, fewer callbacks, stronger outcomes.
